Radioactive iodine has been found in ocean water near the plant at 3,355 times the regulatory limit on Tuesday, CNN reported.
Monitoring data detected the I-131 isotope at excessive levels 330 metres away from one of the plant's discharge points, Japan's Nuclear and Industrial Safety Agency said.
"I do not believe there's an immediate threat to fishery products, as no fishing is being conducted within 20 kilometres of the nuclear power plant," Hidehiko Nishiyama of the agency told CNN.
Radiation levels recorded at a village outside the evacuation zone around the quake-stricken Fukushima nuclear plant are above safe levels, the UN atomic watchdog says.
The International Atomic Energy Agency said safe limits had been exceeded at Iitate village, 40km northwest of Fukushima, well outside the government-imposed 20km exclusion zone and the 30km "stay indoors" zone.
"The first assessment indicates that one of the IAEA operational criteria for evacuation is exceeded in Iitate village," the IAEA's head of nuclear safety and security, Denis Flory, told reporters.
The watchdog had advised Japanese authorities to "carefully assess the situation and they have indicated that it is already under assessment," Flory said.
But he said the IAEA - which does not have the mandate to order national authorities to act - was not calling for a general widening of the exclusion zone.
Advice had been given to "carefully assess the situation and they have indicated that it is already under assessment," he said.
The reading in Iitate was merely a spot reading, he said.
"Deposition of radioactivity is something which is not the same everywhere, it depends on wind, it depends on rain and also on profile of terrain," Flory said.
"Saying at one point that there is a need to assess further does not mean that all around that is a concern."
But he said that overall, the situation at Fukushima "remains very serious."
According to Elena Buglova, head of the IAEA's Incident and Emergency Centre, the reading in Iitate village was 2 megabecquerels per square metre.
That was a "ratio about two times higher than levels" at which the agency recommends evacuations, she explained.
Meanwhile, Fukushima Daiichi plant operator Tokyo Electric Company president Masataka Shimizu has been hospitalised due to "fatigue and stress", company Chairman Tsunehisa Katsumata told reporters yesterday.
Shimizu was last spotted at public at a news conference on March 13, two days after the quake and tsunami, CNN reported.
- AFP, NZ Herald staff